Propeller pitch control



July 29 1947' J. F. HAINESl 2,424,749

PROPELLER PI TCH CONTROL Filed oct. 12, 1942 2 'sheets-sheet 1 zafw ,44 xNvEToR' Y .Jb/m fbames l July 29, 1947.

' J. F. HAlNr-:s

v PROPELLER FITCH? CONTROL Filed Oct. 12, 1942 2 Sheets-Sheet 2 Patented .girly 29, i?

PROPELLER FITCH CONTROL John lF. Haines, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application October 12, 1942, Serial No. 461,686

9 Claims. (Cl. ITU-163) This invention relates to controllable pitch propellers in which a fluid medium under pressure is led to control apparatus that respond to predetermined conditions for effecting a prescribed control of the propelling apparatus.

One object of the invention is to provide a uid control system with means for storing a reserve of fluid pressure to be available during substantial rest of the controlled apparatus.

Another object of the invention is to provide a fluid control system with an accumulator that may be carried with movable mechanism and provide a reserve of energy so as to be constantly available for exercising its potential under any condition of the apparatus.

A further object is to provide an accumulator of toroidal form that can be mounted on and rotate with rotating control apparatus.

A further object of the invention is to provide a propeller control mechanism with a self-contained fluid circuit including an accumulator rotating with the propeller and for storing energy for supplying power for feathering the blades of the propeller, and all without making uid connections to an outside supply.

A further object is to provide an accumulator and control valve assembly for applying the reserve oi power from the accumulator when the pressureinducing means is inactive.

Another object is to provide a control valve for a fluid actuating circuit that is characterized by means permitting a build up of pressure in the supply line to a preselected value, and then operating automatically to unload the system, yet retaining the stored pressure for use at a subsequent time.

Yet another object of the invention is to provide a valve unit for control of a uid circuit by the dominance of either accumulated pressure in the system, or 'the actionof centrifugal force in opposition to the accumulated pressure.

Still another object is to provide a pressure limiting means responsive to "the dominance of centrifugal force or accumulated pressure in marking the maximum of pressure built up in the system.

A further object is to provide a self contained huid circuit for a controllable pitch propeller, with a reserve of uid pressure for accomplishing supplementary control of the blades during relative inaction of the propeller,

Another object is to provide an accumulator by cooperation of parts of the regulator housing 2 scribing annular chamber adapted to rotate with the regulator.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein preferred embodiments of the present invention are clearly shown.

In the drawings:

Fig. l is an elevational view of a controllable pitchpropeller mechanism with parts shown in section to illustrate the relation of parts of the control units, the section being a view substantially, as indicated by the line and arrows i-l of Fig. 5.

Fig. 2 is a cross sectional view of an accumulator, of modified form, adapted for attachment to a standard regulator.

Fig. 3 is a fragmentary view in section illustrating features of accumulator connection substantially as indicated by the line and arrows 3-3 of Fig. 2,

Fig. 4 is a sectional View, enlarged, of the deformable partition for the accumulator.

Fig. 5 is a sectional view transverse of the regulator substantially as indicated by the line and arrows 5 5 of Fig. 1, showing the relation of the parts and the fluid circuit connecting them.

Fig. 6 is an enlarged sectional view of the valve unit, it being a view substantially as indicated by the line and arrows 6 5 of Fig. 5.

Fig. 'l is a modication of iiuid connection to the accumulator.

With reference particularly to the drawings, l@ indicates a propeller shaft having a shoulder I2 against which is seated a rear cone lf3 adapted to take the thrust o-f a shaft nut in securing thereon a propeller hub I6. The hub provides a desired number of sockets i8 in each of which ls pivotally mounted a blade 20 whose pitch setting is effected by a torque applying unit located in the blade root and controlled by fluid actuated means located in a regulator 22 mounted on the propeller hub for rotation therewith. In accomplishing this, the hub IE provides a rearwardly extending sleeve 24 surrounding the shaft and providlnga shoulder 26. The regulator comprises an annular plate 28 surrounding the sleeve and seated against the shoulder 26 where it is secured by means of a sleeve nut 30 threaded upon the hub extension at 32.

The plate 28 forms one wall of a reservoir for enclosing fluid pressure control mechanism and connecting means to form a self contained and a ring member to make use of a circumand unitary assembly. The reservoir, designated by the numeral 34, is completed by a circumscribing wall 38, an annular plate member 40, and an adapter sleeve 42 the latter of which is fixedly supported from outside of the reservoir. The sleeve 42 extends through the plate or cover 40 to bridge the space between it and the plate 28 where it has a more or less socketed engagement, and engages fluid seals -44 at the cover 40, and 46 at the plate 28.

The seals 44 and 46 comprise flexible rings .carried by ledges of the plate and cover and have a flexible portion normally in running contact with the sleeve 42, but are adapted under rotation of the regulator to lift off or reduce pressure upon the sleeve 42 so as to reduce friction and wear of "the seals. Under a state of rest they engage the sleeve 42 firm enough to prevent fluid flow from the reservoir. While rotating, any liquid contained in the reservoir is cast outwardly against the peripherial wall 38 at Which time the seals do not need to be tight against the adapter sleeve.

The fluid charge of the reservoir 34 immerses a substantial portion of each of the units of the control mechanism enclosed within the reservoir and mounted on the plate 28, where they make fluid connection with integrally formed passages. The control mechanism includes a pump or pressure developing unit 48that has an intake 5I) from the reservoir and discharges into a pressure supply passage 52 leading to a pressure limiting valve unit 54, later to be described, and thence to a governor or distributor valve unit 56 by way of a passage 58. From the distributor valve unit the fluid under pressure is led to either side of a torque applying unit 59 (see Fig. 5) in the root of the blade, which leading is accomplished by one or another of the control passages 60 or S2, and transfer tubes 64 and 66.

In a preferred form the tubular passages connecting the various control units are in the form of bent tubesdeslgned and placed so as to con- .hect properly to and from the desired stations where the units are to be placed, and'then are cast as inserts in the body of the supporting plate. Drillings properly located in the casting and opening into the tubes make it feasible for connection of the various units by the clamp-on method, where they may be secured by screw devices 58 and the like. Drilllngs 70 and 12 from the outside of the plate 28 make for reception of the ends of the transfer tubes 54 and 86 when the plate is assembled with the sleeve extension 24 of the hub. Drillings I4 and I6 from the inside face of the plate facilitate connection with the valve unit 54, and drillings 18, 80 and 82 afford proper connection of the governor valve unit with the supply passage 58 and the control passages 80 and 62 respectively.

The pressure creating means 48, that is selected for illustration here, is of the intermeshing gear type or one in which a source of uid under pressure is delivered so long as the gears are under rotation. The pump is driven by means of a spur gear 84 meshing with a fixed gear 8E formed as a flange on the stationary adapterl sleeve 42. Since the gear and sleeve are rigidly supported from the outside, and since the regulator rotates about the sleeve, the pump is caused to revolve about the gear 86 which 4causes rotation of the pump elements and draws in fluid at 50 from the reservoir and pushes it along the passage 52 to the valve unit 54. From the valve unit, to be described in detail later, the fluid under pressure flows through the passage 58 to 4 the drilling 18 where it encounters the governor valve unit 56.

The governor valve unit comprises a chambered block 88 having a porting sleeve 90 with ports 02, 94 and 86 adapted to coincide with the drillings 18, and 82 respectively. A valve plunger 9B slides along the porting sleeve and has lands |00 and |02 spaced for closing off the ports 94 and 96 when the valve is in the mid or equilibrium position. The valve plunger attains and retains its equilibrium position in response to centrifugal force applied to the valve plunger and an actuating lever |04 pivotally connected thereto, and opposed by substantially constant force of a spring |06. The valve plunger is disposed radially of the axis of propeller rotation and tends to move radially outward in response to centrifugal force applied to the valve and its actuating lever. The spring is supported by a ledge |08 extending from the block 88 and engages the intermediate part ||0 of the lever. As a relatively fixed support of the lever there is a movable fulcrum l I2 provided by a movable carriage I|4 that slides on guides I I E fixed with respect to the block 88. Movement of the carriage ||4 along the guides varies the point at which the end of the lever |04 will rock about the'fulcrum and thus changes the relation and the magnitude of the centrifugal force arm and the spring force arm. Ihat change sets up a new or different speed level at which the governor valve will control or assume an equilibrium position.

In order to have selective control of the speed level, or in order to move the fulcrum to a desired position, the carriage is provided with a shoe I I8 that follows along a groove |20 of a control ring |22 surrounding and movable along the fixed adapter Sleeve 42. For the movement of the control ring there are a plurality of screw shafts |24 threaded into the ring and journalled in a thickened portion |26 of the sleeve 42. The screw shafts extend aft of the adapter sleeve to provide pinions |28 that mesh with a ring gear |30 rotatable in a groove between the sleeve 42 and an attaching plate |32 by which the adapter sleeve is retained against rotation. A lever |34 extending from the ring gear |30 provides for manual movement thereof which causes the screw shafts to rotate in unison and thread into and out of the control ring |22, that in turn sliding along the adapter sleeve and thus moving the fulcrum |I2 toward and away from the point of spring pressure.

The pressure supply passage 58 on its way from the valve unit 54 to the governor valve unit 56, communicates with an accumulator in which may be stored fluid pressure for smoothing the operation of the control units, and for providing a reserve of pressure available at times when the fluid pressure developing means is inactive. The regulator housing is designed to provide part of the accumulator structurel in that the peripheral wall 38 is grooved peripherially and exteriorally to provide a channel |38 semiclrcular in cross section surrounding the regulator. A cover ring |38 having an internal groove |40 of similar but complementary semicircular cross section telescopically engages over the peripheral wall 38 where it is held in sealing relation by screw devices |42. A deformable partition |44 is disposed between the members 38 and |38 where it ls anchored against displacement by thickened rims |48 lodged behind fins |48 and |50 of the members 38 and |38 respectively.

That structure normally provides an annular 'passage of circular crs section at the outer bounds of the regulator and substantially concentric with the axis of propeller rotation. The

partition member |44 has a width between the f rims |46 far in excess of the diameter of the cir; cular passage so that when anchored between the members 38 and |38 the intermediate or spanning portion |52 will buckle or bulge to one side. The member |44 thereby divides the annular passage into two chambers, |54 and |56. The chamber |56 may be termed a loading chamber since it is radially outward of the deformable partition and may enclose a volume of air under pressure, or some inert gas. The chamber |54 may be termed a pressure chamber since opening |58 extends from the groove |36 into the pressure passage 88. The cover plate 40, for the reservoir, may be secured to the aft rim of the ring member |38 by screw devices |60.

To improve the stability of the pressure chambl, it may be desirable, to locate the air or loading chamber inward of the pressure chamber as is shown in Fig- 7, where the grooves |36 and |40 separated by the deformable wall |44 provide the loading chamber |156a inward ofA the pressure chamber |54a. The ring member |38a provides a cast in passage |55 into which a plurality of small openings |58a lead from the pressure charnber |54a. Aligned drillings |51 and |59 make communication between |55 and the passage 58a, while a nipple |61 and sealing ring |63 make a fluid tight joint between the plate 28 and ring |88a. In this construction, the pressure chamber |60a is radially outward where the mass of iiuid medium will not be cast against the partition |44 purely by reason of centrifugal force of rotation. l

As the fluid in the system develops pressure it flows into the pressure chamber |54 from the passage 68 to compress the loading gas in the chamber |56. That chamber always being under inflation will constantly exert a force upon the iluid content of the chamber |54 tending to drive it outward through the openings |58, whereby a reserve of fluid pressure is. had even though the pressure developing means 48 is inactive, or insumciently active to deliver fluid under pressure to the fluid circuit.- To insure that the loading of the partition member |44 will not cause rupture of the partition when it is forced against the opposing wall of the pressure chamber |54, the openings |58 leading to the pressure passage 58 are desirably small and many as shown in the drawings. Though the size of the openings is small the number are sufcient to make up an equivalent area of openings that will not materially increase the resistance to fluid flow.

In the form of structure shown in Figs, 1 and "I, the accumulator is of the built-in type but may as well be embodied in an attached or accessory structure somewhat as shown in Figs. 2 and 3. There the accumulator structure comprises a pair of telescopically engaging rings |62);r

and |64 assembled with the partition member |44 to form the loading chamber |56 and the pressure chamber |64. As such a structure, it is insertable between the plate 28 and cover 40 of the regulator, or is adapted to be attached thereto in any convenient manner. In this form the chamber |56 is loaded with air or gas under pressure through an opening |66 similar to the preferred form and in which is secured a Valve of the pneumatic tire type. Similarly, a recess |68 in the body of the ring |62 is bottomed with aper- Ytures |18 that communicate with the pressure chamber |54, and from which, it is understood, fluid connection is lead to the pressure supply passage 58 in the usual manner.

The pressure limiting valve unit 54 is inserted in the iluid path between the pressure developing means 48 and the accumulator, with an object to effect the charge or storing of pressure in the accumulator during operation of the pump, to prevent return ilow of uid under pressure from the accumulator, and to unload the pressure developing means when its delivery reaches a selected pressure value. The valve unit comprises a self-containedunit adapted to be connected into the fluid circuit by the clamp-on method, and comprises a housing member |12 providing a well |14 and a through bore |16. The bore |16 is so formed in the housing as to be radially disposed when the housing is mounted on the plate 28, and is enlarged at the inboard end |18. 'I'he smaller bore |16 has intermediate its length an undercut |80, and the juncture of the large and small bore has an undercut |82. A piston valve |84 is disposed mainly in the bore |16 and is of sufficient length to cover the undercut |80. An enlarged portion |86 lapped into the bore |18 provides a differential or fractional area exposed to the undercut |82, from which a Ibore |88 leads into the well |14.' Attached to the piston valve |84 there -is a weight member |98 that has a pin |92 passing through the valve piston and ends ma spaced head |94, that permits relative movement between the weight and piston. A spring |96 enclosed thereby normally tends to separate the weight and piston, so that the piston valve moves outwardly against the juncture of the two diameters of the bore to cover the undercut |80, and so that the weight moves inwardly to rest against a, stop |98 attached to the housing by screw devices as 200. The well |14 houses a check valve assembly including a cup 202 whose bottom wall is apertured at 204 and supports a pin 206 that retains and guides a disc valve 208 under the urge of a spring 2|0 restingagainst the bottom of the well. 'I'he valvev assembly is held in place by a cover plate 2|2 and screw devices 2l4, and tends to divide the well |14 into a delivery chamber 2|6 and a pressure chamber 2|8, into which the bore |88 from the undercut |82 empties. A drilling 220 opens the pressure chamber 2|8 to the drilling 16 communicating with the pressure passagev 58, while drilling 222 connects the drilling 14 with the delivery chamber 2 |6, and the drilling 224 connects the delivery chamber with the undercut |80.

The valve unit operates as follows. It is unique in that it operates with snap action under iniluence of centrifugal force, and also acts with a graduated effect below critical speed and pressure. In one instance, it performs a pump unloading function, and in another instance it performs a pressure limiting function. During rotation starting from rest, the valve posts will be related as shown in Fig. 6 where the weight and valve member will :be separated and the weight will be urged radially inwardly against the stop |98 by the spring |96 which also pushes the piston |84 outwardly of the stepped bore to close the undercut |80. With the Vpropeller rotating, the pump 48 revolves about the gear 86 which causes rotation of the pump gears to take in uid from the reservoir through the intake 50-and deliver the fluid under pressure to the passage 52, by which it empties into the delivery chamber 2|6 through drillings 14 and 222. Fluid under pressure in the delivery chamber 2|6 flows past position to the other.

the disc valve 208 and into the pressure chamber 2|8. Thence, by way of the drillings 220 and 16, and the passage 58 the fluid under pressure flows to the governor valve port 92, on its way engaging the holes |58, |58a or |10 leading to the pressure chamber |54 of the accumulator. Assuming that all outlets from the governor valve unit are closed. then the pressure in the pressure passage 58 and in the accumulator chamber |54 will build up to a predetermined value, in so doingr compressing the gas in the loading chamber |56. That developed pressure is restrained from returning to the pump by the check valve element 208, but the pressure existent in the accumulator and pressure chamber 2|8 is communicated to the fractional area of the piston |84 by reason of the drillway |88 and the undercut |82.

At low speeds of rotation, such as those less than required to throw the Weight |90 outward by reservoir through 224, |80 and |18, but the accumulated pressure is maintained. During continued rotation of the propeller, the weight member |90 moves from a position of rest against the stop |98, radially outward as the speed of rotation increases. Under those conditions, inward movement of the piston valve is now opposed by both an additional force of the spring |96 (under compression) and centrifugal force acting on the member |84 and the weight member |90, which steps up the magnitude of the force opposing inward movement of the piston valve to open the delivery chamber to the reservoir. Themovement or" the valve when so coupled with the weight |90 is of the snap action type, since the Weight in moving inward or outward changes the elect of centrifugal force thereon, but does not change the opposing force of fluid pressure upon the fractional area of the piston. The disturbance of balanced force on the Valve is sufcient to move the valve and weight quickly from one extreme When the accumulating pressure starts tovmove the piston valve inward it has a magnitude sufiicient to overcome the centrifugal force applicable upon the assembly at a certain radial distance from the axis of ro-v tation, and the inward movement of the weight |90 shortens the moment arm through which centrifugal force acts, thus decreasing the effect of centrifugal force even though the speed of rotation is unchanged. The accumulator pressure remaining unchanged has an even greater advantage over the opposing centrifugal force and moves the valve and weight assembly inward more rapidly.

Thus, under normal propeller rotation above a predetermined speed, the valvel piston |84 is normally urged to close the undercut |80, by reason of the centrifugal force acting upon the valve and weight. While the valve and weight are in this collapsed relation, and at the outboard limit of their movement, they are subject to radial inward movement in response to stored-up pressure in the pressure chamber of the accumulator.

8 That pressure operating from the pressure chamber 2|8 and through the drill way |88 is applied to the fractional area of the piston valve which, when it becomes great enough, causes the valve to snap radially inward against the opposing forces to a position against the stop |98, so that the undercut is completely uncovered. When that occurs. the high pressure limit of the uid circuit'connected with the accumulator is attained, since the fluid under pressure within the delivery chamber 2|8 may then exhaust through the drilling 224, undercut |80 and bore |16 back into the reservoir 34, instead of passing the valve 208 into the pressure chamber 2|8.

The valve unit 54 therefore operates to effect charging of the accumulator with fluid kunder pressure, but prevents the pressure developed from forcing the liquid back through the pump should its delivery fall olf. The pressure developed in the accumulator is kept available until made use of through operation of the governor valve either automatically or manually. On the other hand, should the pressure in the system reach a predetermined high value the valve unit returns all fluid that might result in a higher pressure to the reservoir. y

The pressure developed in the accumulator is always available at the intermediate port 92 of the governor valve unit, and should there be an overspeed condition. the valve plunger 98 in moving radially outward operates to connect that port with the port 94 leading to the control passage 60 and thence through transfer passage 84 to one side of the torque applying unit for increasing the blade pitch. On the other hand, should there be an underspeed condition, then the valve plunger moves inwardly to connect the port 92 and 9E which in a similar way admits fluid under pressure through passages 62 and 68 to the opposite side of the torque applying unit which tends to decrease the pitch of the blade. That control of the blade shift is wholly automatic and tends to control the operation of the propeller at a substantially constant speed, the level of which may be determined by the setting of the fulcrum ||2 through the selected position of the lever |84. Aside from the shift of the blade pitch, by automatic means, it may be necessary or desirable to movably change the pitch setting to a feathered position and back again to a Working range. If an engine fails or stops, the blades of the propeller, unless moved to a particular position, will by reason of the windmill action, cause the engine to continue to turn over, and perhaps cause damage. In the present instance, the manual control of the fulcrum ||2 may be actuated 'to such a position that the fulcrum is on the opposite side of the line of spring force, in which instance the valve plunger is cast radially outward calling for an increased pitch change of the blade. Under those conditions there is no developed resistingforce on the valve plunger and the stored up pressure of the accumulator operates to move the blades tothe feathered or non-windmilling pitch position illustrated by dotted lines in Fig. 5. Subsequently, if it is desirable, the fulcrum of the governor valve may be moved aft toward the end of the lever |04 where the forces of the spring on the lever will be reversed to cause the valve plunger 98 now to be forced inwardly to connect port 92 and 96. The remaining stored up pressure of the accumulator will now be available to elTect sufficient pitch change in the blades to bring them out of the feathered position to a working position shown by full lines in Fig. 5, and allow them 9 to windmill, which may be availed of to crank the engine.

While the embodiments of the present invention as herein disclosed,V constitute preferred forms, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

l, In a system for controlling the pitch of propeller blades by a. hydraulically operated device, the combination of an accumulator for liquid under pressure, resilient means associated with the accumulator for varying the pressure therein in accordance with the amountof liquid present in the accumulator, a reservoir, a pump system connected with the reservoir and including a delivery chamber, means for limiting the pressure in the chamber to a predetermined value, a communication between the delivery chamber and the accumulator, and a check valve mounted in the communication and adapted to open only in the direction of the accumulator, said pressure limiting means comprising a centrlfugally actuated y valve and opposing piston exposed t accumuunload the delivery lator pressure, adapted to chamber to the reservoir when a predetermined pressure value is obtained.

2. In a system for controlling the pitch of propeller blades by a hydraulically operated device, the combination of an accumulator for liquid under pressure, resilient means associated with the .accumulator for varying the pressure therein in accordance with the amount of liquid present in the accumulator, a reservoir, a pump system connected with the reservoir and including a delivery chamber, means for limiting the pressure in the chamber to a, predetermined value, a communication between the delivery chamber and the accumulator, a check valve mounted in the communication and adapted to open only in the direction of the accumulator, said pressure limiting means comprising a. centrifugally actuated valve adapted to move radially outward of the reservoir to close a relief port from the delivery chamber to the reservoir, a piston valve having a fractional portion of its area exposed to accumulator pressure, a centrifuga'lly responsive weight member operable to actuate said valve, and yielding means transmitting the movement of said weight to said valve, whereby the domination of accumulator pressure over centrifugal force moves the piston valve radially inward to unload the pump system to the reservoir through the said relief port.

3. In a system for controlling the pitch of blades of a propeller by hydraulically operated devices, the combination of a hub, a liquid containing reservoir mounted on the hub for rotation therewith, an annular accumulator surrounding the reservoir, control apparat-us carried by the reservoir, and including a continuous delivery pump system having an intake from the reservoir, means connecting the output of the pump system lto the accumulator and for returning the excess of a predetermined pressure in the accumulator to the reservoir` 4. The combination set forth in claim 2, wherein the accumulator includes a deformable partition dividing the annular passage into a loading chamber radially outward of a pressure chamber, and the fluid Connections are constantly open to the pressure chamber,

5. The combination set forth in claim 2, wherein the means providing the accumulator comprises a pair of telescoping ring members sel0 r cured for rotation with the reservoir, said rings being channeled at their proximate portions, and a deformable partition secured between the ring members dividing the accumulator into two substantially concentric chambers.

6. The combination set forth in claim 2, wherein the means providing the accumulator comprises in part a channel in the circumscribing wall of the reservoir, a cover ring telescoping with the channeled part of the reservoir, and a exible diaphragm clamped in sealing relation between the reservoir and cover ring whereby the accumulator is divided into two concentric noncommunicating chambers.

7. In a system for controlling the pitch of blades of a propeller by hydraulically operated devices, the combination of a hub, a liquid containing reservoir mounted on the hub for rotation therewith, an accumulator mounted for rotation with the reservoir, control mechanism carried by the reservoir and having fluid connections with the reservoir and accumulator, said control mechanism including a continuous delivery pump system and a valve unit for supplying'fluid under pressure to the accumulator, said valve unit spilling'an excess of fluid into the reservoir when the pressure in the accumulator reaches a predetermined value.

8. In a system for controlling the pitch of blades of a propeller by hydraulically operated device/s, the combination of a hub, having pitch shiftable blades, hydraulic devices for shifting the blades, a liquid containing reservoir mounted on the hub for rotation therewith, an accumulator of annular form concentric with and mounted for rotation with the reservoir, control mechanism mounted within the reservoir and having fluid connections with the reservoir and accumulator, speed responsive means exposed to the fluid pressure in the accumulator and adapted to selectively distribute that pressure to the .hydraulically operated devices for eifecting a change of pitch in the propeller blades in response to speed of propeller rotation, said accumulator pressure being sufficient and available to shift the blade pitch to a feathering position, and control means operable upon the speed responsive means for selectively applying the accumulator pressure to the hydraulically operated devices.

9. A system for controlling the pitch of propeller blades by a hydraulically operated device comprising in combination, a hub having a rotatable plate, a pressure developing means supplying fluid under pressure to the system, an accumulator charged with fluid under pressure in excess of that required for the normal operation of said device, a centrifugally operated valve unit for limiting the pressure of the system, comprising a block mounted on the rotatable plate, and having uid connection with said pressure developing means and said accumulator, a piston valve and weight member loosely coupled for relative axial movement, spring means normally expanding the piston4 and valve toclose a relief port from said pressure developing means, said piston valve having a fractional area exposed to the accumulator pressure for gradually opening said relief port while the propeller is rotating at a speed below a critical speed and while the accumulator pressure is below a critical pressure,-

and means including the collapse of said spring means and relative axial movement of the weight and valve for increasing the resistance to movement of said valve by accumulator pressure, whereby said valve and weight will move as a unit with snap action to control uid passage through said relief port.

JOHN F. HAINES.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,712,791 Gallagher May 14, 1929 2,214,817 Harrington Sept. 17, 1940 2,296,288 Martin et a1 Sept. 22, 1942 Number Number Name Date Johnson Aug. 6, 1940 Thornhill et al. Apr. 28, 1942 Blanchard et al Jan. 5, 1943 Rindeisch Sept. 30, 1941 Herma-n July 20, 1943 Indrieri Feb, 25, 1941 Mercier Apr. 22, 1941 Rindesch May 25, 1943 FOREIGN PATENTS Country Date Germany Feb. 6, 1941 Great Britain Feb. 25, 1938 France July 3, 1931 

